The growth of carbon nanotubes (CNT) on metal substrates (CNTM) has been a subject of interest for the last two decades because of the immense potential of CNTM as functional materials for filtration, sensing, energy storage, and heat transfer (References 1-9). Carbon nanotube on metal substrate materials have been developed on sheet or powder substrates or mesh screens with large openings (References 4, 10, and 11). A substantial number of studies have been conducted to understand the CNT growth mechanism, characterize the quality of CNT, and evaluate potential applications of CNTM by considering their mechanical and electrical properties (References 9 and 11-16). However, to the best of our knowledge, the development of CNTM on suitable porous or mesh screen substrates, as a method for membrane fabrication, has not been investigated.
Carbon nanotubes grown on metal substrates can have superior hydrophobicity, which is a critical property for many applications, such as membrane distillation and condensation heat transfer (References 17 and 18). It is also critical for the grown CNT in the CNTM material to maintain its interfacial bonding and resist delamination when the materials are exposed to high humidity, corrosive gases, or corrosive conditions in aqueous solutions. A limited number of reports have been published on the surface wettability of CNTM materials, but no work has been identified that evaluates the performance and stability of CNTM in humid conditions or corrosive environments (References 5, 10, 19, and 21). For example, De Nicola and co-workers reported that multiwalled CNT grown on stainless steel (SS) have a superhydrophobic property (water contact angle of 154°), and Zhang and Resasco showed that aligned single-walled CNT are superhydrophobic (References 20 and 21). The deposition of hydrophobic coatings, such as Teflon, gold thiol, and silicone, on top of the CNT forest has also been reported to make the forest superhydrophobic (References 22, 23, and 24). However, the thermal or chemical stability of these or other superhydrophobic CNTM materials for practical applications has not been investigated.
Hydrophobic CNT is a material of interest for water desalination because of the unique nanoscale interactions occurring along the graphitic walls, leading to fluxes several orders of magnitude higher than values predicted by continuum hydrodynamics theory (References 25, 26 and 27). Membrane distillation, a desalination technology that purifies water by allowing only water vapor to pass through hydrophobic pores, can benefit greatly from the availability of suitable CNT membranes with high hydrophobicity, enhanced flux, and resistivity to microbial fouling (Reference 28). Even though the commercial potential of CNT membranes has been shown, commercially viable, robust CNT membranes have not yet emerged. The higher manufacturing cost involved with micro- and nanofabrication of CNT membranes, the health hazards caused by the presence of CNT in a permeate after dislodging (because of weak interfacial bonding) from the membrane, and the stability of the CNT under realistic application conditions (e.g., high temperature, an oxidative atmosphere, or a corrosive environment) are some of the major challenges involved with producing a viable CNT membrane (Reference 28).